My latest microphone preamp design is completed and sounding very cool… The Old ’76.
The Old ’76 is a novel three-stage design; it’s not based on any past or current production microphone preamp. That being said, the circuit is nothing revolutionary: an input transformer (in this case a UTC O-1, as I was temporarily out of Jensen 115s) connects to an RCA 76 tube, biased in textbook (or, at least, RCA-Receiving-Tube-Manual-book) fashion; volume pot follows and then on to a 6SL7 tube with the first stage plate directly coupled to the grid of triode 2; the output is a cathode follower with a Solen cap and onto an Edcor output transformer. The pushbutton switch (with associated pilot lamp) activates 48V phantom power. Overall gain is approx. 60db and response is flat 50hz – 18khz. I am betting that the slight high and low end roll off is due to the UTC O-1; the next build will use my customary Jensen input transformer and we should see 25hz-20khz flat response.
76 tubes were often used in ancient console radios with a large tubular shield positioned around them; the reason for this became quiet clear once I had finished my piece; before I added the above-depicted aluminum plate between the 76 and 6SL7, I was getting some unpleasant ringing on very high frequencies.
I had wanted to build a mic pre with some of these very old two-digit designation tubes for some time now. Hi-Fi fans seem to love the 76 tube, so I figured there was probably something worth investigating. 76 tubes are readily available and pretty inexpensive; it is a bit of a challenge to find the 5-pin bases that they require though. AES sells only one 5-pin base, and it’s expensive and oversized. My prototype unit here used an older Amphenol socket that I dug up somewhere.
Some other miscellaneous design notes: B+ rectifier is a 6×5 tube; hammond 15mh choke was used in the B+ chain; DC filaments, as usual; the phantom power supply has a slow ramp-up when activated; the three-pole phantom switch 1)connects the 48v supply to the main power supply; 2)connects the phantom indicator lamp to the 6v supply; and 3) connects the 48v to 6.8k resistors that actually connect to the mic input jack. I find a three-pole switch necessary in these instances because there is some ‘drain’ time involved when phantom is turned off, even with a bleeder resistor; disconnecting the 48v right before it hits the mic jack provides 100% assurance that you (I) won’t melt that BK5 ribbon (again).
An A/B test of The Old ’76 vs. the usual API 512 will be posted here in the next month or so.
Hi Chris,
Indeed, these ancient triodes are wonderful, obscure candidates and often overlooked when building gear.
Usually people rely upon the 12AX7/12AU7 range or their octal-based cousins, the 6SL7/6SN7 variety. But I like the old 5-pin based triodes a lot better.
A while ago I built a line-driver with the RCA UY-227 to be part of my monitoring chain. The 227 (or 27) was the first series-produced indirectly-heated triode and introduced to the public in 1927. It has a fat globe-shaped bulb and a mesh plate. Beautiful RCA logo on top. Quite a sight.
And then there is the sound. It just blows you away. If you hear what this dinosaur can do to, say, the tone of a Martin 6-string, or any other decently recorded acoustic, it’s just breath-taking. Setting up a three-dimensional soundstage is another forte of this tube. In fact there’s no going back to a 12AU7 or even a 6SN7 once you’ve been exposed to the 27; they sound dull and gray by comparison.
The only downside I can think of with the 27 is that some samples can be microphonic, so isolating them and shielding is a good idea.
In getting the best from these tubes sound-wise, it’s mandatory that the rest of the components perform at the same level, especially the coupling caps between stages.
Here I always use paper-in-oil types for ultimate smoothness. Really pays off.
Love your site!
All the best,
Kees